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Re: Problem Using a Commodore 1084s Monitor on a IIgs

Polymorph wrote:

On Feb 11, 6:32 am, "Michael J. Mahon" <mjma...@aol.com> wrote:


Polymorph wrote:


Now, as you can see from the above images, it doesn't appear to be
fully synched. All the characters are leaning to the right (like
everything is in italics), and you can see the black area on the RHS
of the screen which is wobbling around a bit.


This means that the horizontal sync is not quite right, either.
You could adjust the horizontal sync on the monitor to get it
closer, or....



Michael, as per all of the IIgs-->1084 FAQs suggest, I've only
connected the composite sync from the IIgs to the composite sync of
the 1084. Also note that in the 1084 the composite sync and horizontal
sync are tied together (the 1084S-D schematic I found confirms this).
I have left the vertical sync not connected as per the Apple II FAQ.


I wasn't referring to the horizontal sync input, but to the horizontal
sync (or "hold") adjustment on the monitor.  This may be internal, or
accessible only through use of an "alignment tool" through a hole in
the back or bottom of the case.  Ideally, it would be an external
control like the vertical hold control.


I'm thinking that in my case, 75 Ohms is not quite right, but
something in the vacinity would be right. Does this sound plausible?
Do you recommend that I buy a variable resistor and try and find the
optimum value? Or should I being trying something else.


Exactly.  Try lowering the value (or, better, adding a 75 to 150 Ohm
pot with the IIgs driving the "high" end and the output to the monitor
coming from the center terminal) until you can find where the sync
drops out because the level is too *low*.  Then increase it until good
lock is achieved.



Just so I am clear, when you say "lowering the value" do you mean the
voltage on the composite sync - i.e. lowering the resistor value used?
If that is the case, wouldn't I need a resistance of less than 75
Ohms, meaning a 75 to 100 Ohm pot would be too much resistance? Please
forgive me if I'm talking rubbish.  ;-)

I took a look at the catalog of my local electronics dealer (Jaycar),
and they have a variable 0-200 Ohm variable resistor (this is the
smallest variable resistor they have).  I'm thinking one of these
would fit the bill, what do you reckon?


A 200 Ohm pot is perfect.  Actually, I hope it's a carbon pot, not
a wirewound pot.  Low resistance pots may be wirewound, and the
inductance of the winding can distort high frequency pulse waveforms.

Even if it is wirewound, you can try it.  Or you can go for a carbon
pot of up to 500 Ohms--I don't think the value will be critical in
this application.

In my "better" scenario, the pot is connected like a volume control
between the IIgs composite sync output and the monitor sync input.
A pot is a 3-terminal device--a resistor with a sliding tap.  The
two outside terminals are the "ends" of the resistor, and the middle
terminal is the sliding tap.

You would connect the counterclockwise terminal (the "end" terminal
nearest the sliding tap when the pot is adjusted fully counterclockwise)
to ground (common to both the monitor and the IIgs).  The other "end"
terminal (the clockwise terminal) would connect to the IIgs composite
sync output.  The middle terminal (the sliding tap) would connect to
the monitor sync input.

Then you can adjust the sync input to the monitor from zero (fully
counterclockwise) to maximum (fully clockwise).


You may still find it worthwhile to adjust the vertical and horizontal
hold controls to the middle of their stable ranges.  It seems that
this monitor's settings may have drifted a bit.



The vertical hold was very difficult to adjust to get the image to
stop rolling - i.e. there is a *very* small stable range for the
vertical hold. But it was possible, as the images I posted previously
confirm. I am guessing with a different resistor value this problem
will rectify itself. This problem with vertical hold was only present
using the RGB port however, when I hooked a DVD player up to it via
the composite ports I had no such problem.


Interesting.  I wonder what the sync output spec on the IIgs is?
Could it be TTL levels: 5v peak-to-peak?  If that's the case, no
wonder the level needs to be reduced, since most monitors would
expect 1v or less peak-to-peak.

Hopefully, you will find a setting (guessing around 20%) that will
sync stably for both horizontal and vertical scan.

There is a possibility that the monitor expects the sync signal to
be opposite in polarity to what the IIgs outputs.  If that is the
case, an inverter before the pot will straighten things out.

Needless to say, some definitive specs for the sync signal that
the IIgs produces and what the monitor expects would answer all
these questions.


What would you guys recommend as the next course of action? All
comments welcome.


You're almost there!



Great! I was hoping you'd say that.   ;-)


If it "almost works" then you can't be far from having it work
perfectly.

-michael

NadaPong: Network game demo for Apple II computers!
Home page:  http://members.aol.com/MJMahon/

"The wastebasket is our most important design
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